The stratum corneum is chiefly responsible for the barrier properties of skin. Thus, it is this layer that presents the greatest barrier to transdermal flux of drugs or other molecules into the body and of analytes out of the body. The stratum corneum, the outer horny layer of the skin, is a complex structure of compact keratinized cell remnants separated by lipid domains. Compared to the oral or gastric mucosa, the stratum corneum is much less permeable to molecules either external or internal to the body. The stratum corneum is formed from keratinocytes, which comprise the majority of epidermal cells that lose their nuclei and become corneocytes. These dead cells comprise the stratum corneum, which has a thickness of only about 10–30 microns and, as noted above, is a very resistant waterproof membrane that protects the body from invasion by exterior substances and the outward migration of fluids and dissolved molecules. The stratum corneum is continuously renewed by shedding of corneum cells during desquamination and the formation of new corneum cells by the keratinization process.
Historically, drugs have been delivered across the skin by injection. However, this method of administration is inconvenient and uncomfortable, and is not suited for self-administration by members of the general public. Additionally, used needles continue to pose a hazard after their use. Therefore, transdermal drug delivery to the body is particularly desired.
There are many techniques known in the art for transdermal drug delivery and monitoring applications. One well-known example of the need in the art for less painful puncturing of a biological membrane is in the field of diabetes monitoring. The current standard of care for a patient with diabetes includes a recommendation of 3 to 5 painful finger-stick blood draws per day to allow them to monitor their blood glucose levels. Other than the relative size of the lancets decreasing over the last few years, the use of lancets, and the resulting finger sensitivity and pain, has not changed for many years.
To enhance transdermal drug delivery, there are known methods for increasing the permeability of the skin to drugs. For example, U.S. Pat. No. 5,885,211 is directed to thermal microporation techniques and devices to form one or more micropores in a biological membrane and methods for selectively enhancing outward flux of analytes from the body or the delivery of drugs into the body. PCT WO 00/03758, published Jan. 27, 2000 is directed to methods and apparatus for forming artificial openings in a selected area of a biological membrane using a pyrotechnic element that is triggered to explode in a controlled fashion so that the micro-explosion produces the artificial opening in the biological membrane to a desired depth and diameter. PCT WO98/29134, published Jul. 9, 1998 discloses a method of enhancing the permeability of a biological membrane, such as the skin of an animal, using microporation and an enhancer such as a sonic, electromagnetic, mechanical, thermal energy or chemical enhancer. Methods and apparatus for delivery or monitoring using microporation also are described in PCT WO 99/44637, published Sep. 10, 1999; U.S. Pat. No. 6,022,316; PCT WO 99/44508, published Sep. 10, 1999; PCT WO 99/44507, published Sep. 10, 1999; PCT WO 99/44638, published Sep. 10, 1999; PCT WO 00/04832, published Feb. 3, 2000; PCT WO 00/04821, published Feb. 3, 2000; and PCT WO 00/15102, published Mar. 23, 2000.
There remains a need for improved methods and devices for transdermal delivery of agents such as drugs and monitoring of analytes such as blood components.